The continuous shrinking in dimensions of electronic devices utilized in ULSI circuits in recent years has resulted in increasing the resistance of the BEOL metallization as well as increasing the capacitance of the intralayer and interlayer dielectric. This combined effect increases signal delays in ULSI electronic devices. In order to improve the switching performance of future ULSI circuits, low dielectric constant (k) insulators and particularly those with k significantly lower than silicon oxide are needed to reduce the capacitances.
Most of the fabrication steps of very-large-scale-integration (“VLSI”) and ULSI chips are carried out by plasma enhanced chemical or physical vapor deposition techniques: The ability to fabricate a low k material by a plasma enhanced chemical vapor deposition (PECVD) technique using previously installed and available processing equipment will thus simplify its integration in the manufacturing process, reduce manufacturing cost, and create less hazardous waste. U.S. Pat. Nos. 6,147,009 and 6,497,963 assigned to the common assignee of the present invention, which are incorporated herein by reference in their entirety, describe a low dielectric constant material consisting of elements of Si, C, O and H atoms having a dielectric constant not more than 3.6 and which exhibits very low crack propagation velocities.
U.S. Pat. Nos. 6,312,793, 6,441,491 and 6,479,110 B2, assigned to the common assignee of the present invention and incorporated herein by reference in their entirety, describe a multiphase low k dielectric material that consists of a matrix composed of elements of Si, C, O and H atoms, a phase composed mainly of C and H and having a dielectric constant of not more than 3.2.
Ultra low k dielectric materials having a dielectric constant of less than 2.7 (and preferably less than 2.3) are also known in the art. Key problems with prior art ultra low k SiCOH films include, for example: (a) they are brittle (i.e., low cohesive strength, low elongation to break, low fracture toughness); (b) liquid water and water vapor reduce the cohesive strength of the material even further. A plot of the cohesive strength, CS vs. pressure of water, PH2O or % humidity, which is referred as a “CS humidity plot”, has a characteristic slope for each k value and material; (c) they tend to possess a tensile stress in combination with low fracture toughness, and hence can tend to crack when in contact with water when the film is above some critical thickness; (d) they can absorb water and other process chemicals when porous, which in turn can lead to enhanced Cu electrochemical corrosion under electric fields, and ingress into the porous dielectric leading to electrical leakage and high conductivity between conductors; and (e) when C is bound as Si—CH3 groups, prior art SiCOH dielectrics readily react with resist strip plasmas, CMP processes, and other integration processes, causing the SiCOH dielectric to be “damaged” resulting in a more hydrophilic surface layer.
For example, the silicate and organosilicate glasses tend to fall on a universal curve of cohesive strength vs. dielectric constant as shown in FIG. 1. This figure includes conventional oxides (point A), conventional SiCOH dielectrics (point B), conventional k=2.6 SiCOH dielectrics (point C), and conventional CVD ultra low k dielectrics with k about 2.2 (point D). The fact that both quantities are predominantly determined by the volume density of Si—O bonds explains the proportional variation between them. It also suggests that OSG materials with ultra low dielectric constants (e.g., k <2.4) are fundamentally limited to having cohesive strengths about 3 J/m2 or less in a totally dry environment. Cohesive strength is further reduced as the humidity increases.
Another problem with prior art SiCOH films is that their strength tends to be degraded by H2O. The effects of H2O degradation on prior art SiCOH films can be measured using a 4-point bend technique as described, for example, in M. W. Lane, X. H. Liu, T. M. Shaw, “Environmental Effects on Cracking and Delamination of Dielectric Films”, IEEE Transactions on Device and Materials Reliability, 4, 2004, pp. 142-147. FIG. 2A is taken from this reference, and is a plot illustrating the effects that H2O has on the strength of a typical SiCOH film having a dielectric constant, k of about 2.9. The data are measured by the 4-point bend technique in a chamber in which the pressure of water (PH2O) is controlled and changed. Specifically, FIG. 2A shows the cohesive strength plotted vs. natural log (ln) of the H2O pressure in the controlled chamber. The slope of this plot is approximately −1 in the units used. Increasing the pressure of H2O decreases the cohesive strength. The region above the line in FIG. 2A, which is shaded, represents an area of cohesive strength that is difficult to achieve with prior art SiCOH dielectrics.
FIG. 2B is also taken from the M. W. Lane reference cited above, and is similar to FIG. 2A. Specifically, FIG. 2B is a plot of the cohesive strength of another SiCOH film measured using the same procedure as FIG. 2A. The prior art SiCOH film has a dielectric constant of 2.6 and the slope of this plot is about −0.66 in the units used. The region above the line in FIG. 2B, which is shaded, represents an area of cohesive strength that is difficult to achieve with prior art SiCOH dielectrics.
It is known that Si—C bonds are less polar than Si—O bonds. Further, it is known that organic polymer dielectrics have a fracture toughness higher than organosilicate glasses and are not prone to stress corrosion cracking (as are the Si—O based dielectrics). This suggests that the addition of more organic polymer content and more Si—C bonds to SiCOH dielectrics can decrease the effects of water degradation described above and increase the nonlinear energy dissipation mechanisms such as plasticity. Addition of more organic polymer content to SiCOH will lead to a dielectric with increased fracture toughness and decreased environmental sensitivity.
It is known in other fields that mechanical properties of some materials, for example, organic elastomers, can be improved by certain crosslinking reactions involving added chemical species to induce and form crosslinked chemical bonds. This can increase the elastic modulus, glass transition temperature, and cohesive strength of the material, as well as, in some cases, the resistance to oxidation, resistance to water uptake, and related degradations. These crosslinked bonds can be folded, such that under tensile stress they can support some amount of elongation of the molecular backbone without breaking, effectively increasing the fracture toughness of the material. One most famous example is the “vulcanization” of natural and synthetic rubber by the addition of sulfur or peroxide and curing, as invented by Charles Goodyear and independently by Thomas Hancock. When sulfur or peroxide are added to gum rubber, often with an aniline or other accelerator agent, and then the material is cured under heat and pressure, the sulfur forms folded or slanted polymer crosslinks between the polymer strands, binding them together elastically. The result is a greatly strengthened material with increased cohesive strength, and high resistance to moisture and other chemistries. Vulcanization has essentially enabled the ubiquitous use of rubber in many worldwide applications and industries.
In view of the above drawbacks with prior art low and ultra low k SiCOH dielectrics, there exists a need for developing a class of SiCOH dielectrics, both porous and dense, having a dielectric constant value of about 3.2 or less with a significantly increased cohesive strength vs. k curve that lies above the universal curve defined in FIG. 1 For the particular case in FIG. 1, the fracture toughness and the cohesive strength are equivalent. There further exists a need for developing a class of SiCOH dielectrics, both porous and dense, with specific forms of C bonding, possibly including Si—S, S—S and S—CH bonding, with greater organic character, increased resistance to water, particularly within the shaded regions of FIGS. 2A and 2B, and favorable mechanical properties that allow for such films to be used in new applications in ULSI devices.